1. Field of the Invention
The present invention relates generally to wireless telecommunication networks and, more specifically, to locating a mobile telephone handset or other mobile user equipment with precision within a relatively small or indoor region.
2. Description of the Related Art
It is desirable for a wireless telecommunication network to be able to determine the location of a mobile telephone handset or other mobile user equipment (UE) with greater than cell precision. In other words, it is desirable to be able to determine not only in which cell the UE is located but also the location of the UE within the cell. For example, a major concern is providing an emergency calling service that is similar to or compatible with that which has long been provided through the (“landline”) public switched telephone network (PSTN). A person having a landline phone need only dial a short, easily remembered number (e.g., “911” in the United States and certain other countries) for the PSTN to automatically connect the caller with an emergency operator at a public safety answering point (PSAP). The PSAP equipment also automatically determines the caller's location (typically by street address) using an Automatic Location Identification (ALI) database and displays the location on a computer screen for the operator, who can then dispatch emergency personnel to the caller's location. This system is known in the United States as “Enhanced 911” or “E911.” Cellular networks are increasingly providing similar or compatible ALI means for determining the location of a UE in the event of an emergency call. In the United States, the federal government has mandated a system known as Wireless Enhanced 911 for this purpose.
In accordance with Wireless Enhanced 911 and similar requirements, a number of ALI methods have been developed for wireless telecommunication networks to determine the location of a UE with greater than cell precision. These include Assisted GPS (A-GPS), which uses the Global Positioning System of satellites, as well as triangulation methods such as Time Difference of Arrival (TDOA) and Angle of Arrival (AOA), which use comparisons between signals received from the UE at a plurality of cell site radio towers. The device that performs these methods is known by various names, depending upon the network technology, but in some GSM networks is referred to as a Serving Mobile Location Center (SMLC).
The above-described ALI methods suffer from a number of problems and deficiencies. For example, they do not work well within buildings, which are made of materials that reflect RF signals. Also, there is no adequate means for determining the altitude or height at which the UE is located, as triangulation methods are generally limited to latitude and longitude dimensions (x-axis and y-axis coordinates, respectively). Accordingly, it would be desirable to provide a method for determining the location of user equipment with sufficient accuracy to locate a caller in three dimensions, such as within a multi-story building or similar structure. It is to the provision of such a method and system that the present invention is directed.